1. Field of the Invention
This invention relates to antennas and more particularly to an improved electronic pulse scan antenna of the waveguide type which is especially suitable for use in radar applications in the gigahertz region of the frequency spectrum.
2. Description of the Prior Art
The development of small, compact radar systems for use in tanks, terminally guided weapons and remotely piloted vehicles has created a need for a low cost, compact electronic phase scan antenna of the waveguide type which is of small size and weight. The antenna beam should be swept or "steered" electronically to eliminate the need for bulky and cumbersome mechanical scanning systems. Since the antenna is frequently fed by either dielectric waveguide, which is compatable with dielectric-based, millimeter wave integrated circuits, or the older, conventional hollow matellic waveguide, the antenna should be suitable for use with both types of waveguides. Apart from the foregoing military uses, antennas of this type may be used with small radar systems for small boats and light aircraft where size and weight are also a problem.
An antenna which meets many of the foregoing requirements is shown and described in copending U.S. patent application Ser. No. 640,183 which was filed July 2, 1984 by Richard A. Stern and Richard W. Babitt, two of the inventors of the present application, and which was assigned to the assignee of the present application. This antenna comprises a ferrite rod having a longitudinally-extending series of longitudinally-spaced apart perturbations along a first side of the rod which are adapted to radiate electromagnetic wave energy when the ends of the rod are coupled to a source of such energy. The "perturbations" essentially create irregularities in the length of the rod and may take the form of small openings or narrow slots in the side of the rod. Such an antenna operates on the so called "leaky-wave" principle so that the energy radiated from each perturbation is radiated in a direction which is normal to the point of penetration of the perturbation in the rod side. The radiated energy, however, is also radiated from a second rod side which is oppositely-disposed with respect to the first rod side. The third and fourth sides of the ferrite rod are provided with thin metallic plates or shims which are separated from the adjacent rod side by a thin substrate member fabricated of a plastic having a low dielectric constant. Magnetic biasing means, such as a magnetizing coil which is helically disposed along the length of the ferrite rod and metallic plate assembly, for example, are provided to apply a magnetic field along the longitudinal axis of the rod. The magnetic field created by the biasing coil magnetizes the ferrite which causes a change in electrical length of the rod which in turn produces a reciprocal phase shift in the rod. Essentially, the metallic plates on the third and fourth sides of the rod suppress the Faraday rotation of the wave within the rod and cause the electromagnetic beams radiated from the first and second rod sides to be scanned or swept.